Multiple-user multiple-input multiple-output (MU-MIMO) systems can transmit and receive signals to/from multiple users at a single antenna array at the same time. In a MU-MIMO system, multiple signals are sent in parallel and are kept separate from one another by transmitting (or receiving) each signal in a different (e.g., orthogonal) direction or in a different spatial channel. The process of transmitting (or receiving) a signal in a specific direction using an antenna array is known as beamforming. By selecting a beam for each signal that will produce limited interference with other parallel beams, multiple signals can be transmitted or received at the same time. Beams are often calculated to be orthogonal to one another in order to minimize the interference between the beams.
There are two different types of beamforming systems: implicit and explicit. In implicit beamforming the access point calculates the beamforming matrix based on reciprocal channels. That is, the access point receives uplink signals and based on the reception quality and direction of the uplink signals, the access point determines the direction and other parameters for the downlink beams. In explicit beamforming the access point sends out a sounding signal to the mobile stations. The mobile stations then send feedback to the access point regarding the quality of the received signal. The access point can then use the feedback to generate beams for future downlink signals.